Are new main, rod and head bolts necessary?

On performance engines? Absolutely! Just remember this; the ONLY thing holding your engine together, ( the rod caps in place, the heads on the block and the crank in the main caps, and so on) are the bolts. Use old, worn, rusted or cheapie bolts and they'll stretch or let-go. When bolts stretch, things get loose and start to move or “walk”. When things “walk“, failure is eminent! It’s pretty obvious what happens when bolts snap, so I don’t even need to go there.

Look at the image and see what happens when a perfectly good looking bolt loses its temper, or hardening. It stretches! Yes, those are the SAME two bolts from the same type of rods. The one on the right was ran for a few seasons of hard use. It too "looked" just fine to the naked eye before it went back into the rod after a freshen-up. I mean, look at it. It's clean, it's shiny, and the threads are perfect, just like the bolt on the left, except it was fatigued! When it came time to torque it back to spec in the rod, it just couldn't get to its final torque value because it began to stretch, and stretch, and stretch... until it elongated and got super thin in the middle. Again, your eyes can't see metal fatigue, loss of temper, (hardening), or small amounts of stretch that weaken it.

I can’t tell you how many “bargain priced” magazine ad engines we get in here (that need re-doing because they were cheapie engines to begin with) that have old, re-used bolts in them. A bolt might “look” OK, but what do you have... X-ray vision or something? Can you tell fatigue or excessive stretching by simply looking at a bolt? Hardly! The ONLY way to see if a bolt is strong or not is to test it in a Rockwell tester, and the ONLY way to test something in a Rockwell tester is to stress it until it breaks, and then say, “Gee, that bolt was strong”, or “Gee, that bolt sure was weak”. So testing a bolt isn’t exactly an option, so new, strong bolts, such as ARP or someone like that, are a must in any performance engine. And as you can see, even the best quality bolts, such as the ARP rod bolts in the image, can (and will) fatigue, or over-stretch sooner or later, so even after a couple of freshen-ups it's a good idea to "can" all of the old bolts and put some new ones in.